The invention relates to a method for measuring a body having at least one semitransparent layer by applying light escaping from at least one light guide via the front thereof to the body as well as recording at least one area of the body by means of an optical sensor, such as a camera. The invention relates in particular to a method for detecting alterations on at least one tooth in the area of the dentine and/or tooth enamel, in particular in order to detect caries or cracks in the enamel, by applying light escaping from at least one light guide via the front thereof to the tooth as well as recording at least one area of at least one tooth by means of an optical sensor, such as a camera, and subsequently evaluating the image or images determined by the sensor.